fix(snapshot): set entry expiration info in set_ledger_info

[ElliotFriend]

I've noticed some inconsistencies with snapshot creation and use where the various *_entry_expiration values are not set properly when create an Env from a snapshot that did have those values set.

This could possibly be related to #1101, but I'm not sure, since Leigh seemed more focused on prng.

Refs: #1101

What

This commit updates the ledger-snapshot crate to also set the various ledger entry expiration values, if they are set on the snapshot being set in the environment.

Why

When trying to load a ledger snapshot that I've created from an Env with valid entry expiration values, the new Env uses the defaults (which turn out to be 0) for those expiration values.

I noticed trouble trying to run env.register_contract() or env.register_stellar_asset_contract() in a test file after loading a previously valid snapshot, and receive the error: trying to bump past max expiration ledger. (That expiration ledger, being 0 ledgers from now.)

Known limitations

It's possible this is deliberate, and I'm misunderstanding how to use snapshots. If that's the case, we should probably bolster documentation around ledger snapshots to communicate that.

[leighmcculloch]

@ElliotFriend heads up there's another related issue to restoring from snapshots that I'm fixing:

• Env::from_snapshot no longer creates an Env consistent with Env::default #1101